1. Field of the Invention
The present invention relates to an LPP (laser produced plasma) extreme ultra violet (EUV) light source device for generating extreme ultra violet light to be used for exposing semiconductor wafers or the like.
2. Description of a Related Art
As semiconductor processes become finer, the photolithography has been making rapid progress to finer fabrication, and, in the next generation, microfabrication of 100 nm to 70 nm, further, microfabrication of 50 nm or less will be required. For example, in order to fulfill the requirement for microfabrication of 50 nm or less, the development of exposure equipment with a combination of an EUV light source of about 13 nm in wavelength and a reduced projection cataoptric system is expected.
As the EUV light source, there are three kinds of an LPP (laser produced plasma) type using plasma generated by irradiating a laser beam to a target, a DPP (discharge produced plasma) type using plasma generated by discharge, an SR (synchrotron radiation) type using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity near black body radiation can be obtained because plasma density can be considerably made large, light emission of only the necessary waveband can be performed by selecting the target material, and an extremely large collection solid angle of 2π sterad can be ensured because the light source is a point source having substantially isotropic angle distribution and there is no structure such as electrodes surrounding the light source. Accordingly, the LPP EUV light source device is thought to be predominant as a light source for EUV lithography requiring power of several tens of watts.
In the LPP EUV light source, EUV light is generated in the following manner. A target material such as xenon (Xe) is injected by using an injection nozzle into a chamber (vacuum chamber) provided with a vacuum pump. When a laser beam outputted from a laser located outside of the chamber is collected and irradiated to the target, the target turns into plasma and EUV light near 13.5 nm is generated from the plasma.
As the state of the target material, although any one of gas, liquid and solid can be used, a liquid target is thought to be advantageous considering that it is better in EUV light generation efficiency than gas and it is less likely to contaminate the interior of the chamber than solid. Further, as methods of injecting the liquid target, there are a method of forming jets by continuously injecting the target material from the injection nozzle and a method of forming droplets by injecting the target material from the injection nozzle at predetermined intervals. The latter case has advantages that the EUV light generation efficiency can be increased by timing the dropping intervals of droplets and irradiation intervals of laser beams and the contamination within the chamber can be suppressed by reducing waste target materials that are not turned into plasma.
As a method of forming a target of droplets, there is a continuous jet method of providing vibration to an injection nozzle for injecting jets at predetermined intervals. In the LPP EUV light source adopting the method, a vibrator for providing vibration to the injection nozzle is provided. H. M. Hertz et al., “Debris free soft x ray generation using a liquid droplet laser plasma target”, U.S., SPIE, Vol. 2523, pp. 88–93 discloses a structure employing a piezoelectric element as a vibrator. U. Schwenn et al., “A continuous droplet source for plasma production with pulse lasers”, U.K., Journal of physics E: Scientific Instruments, Vol. 7, 1974, pp. 715–718 discloses a structure employing a magnetic coil as a vibrator.
Further, Japanese Patent Application Publication JP-P2004-6365A discloses an injection nozzle for extreme ultra violet light source wherein the injection nozzle includes a target material chamber having an orifice for ejecting a stream of droplets of a target material from the orifice, and a drift chamber consistent with the orifice and for receiving the stream of droplets, the drift chamber being formed with a drift chamber opening having a predetermined length for tolerating the freeze of the droplets when the droplets propagate through the drift chamber and located oppositely to the target material chamber so as to discharge the droplets therethrough.
Further, Japanese Patent Application Publication JP-P2004-31342A discloses a laser plasma extreme ultra violet radiation source including an injection nozzle having a supply source end and an outlet end with an orifice having a predetermined diameter for ejecting a stream of droplets of a target material, a target material excitation source for supplying a pulsating excitation signal to the injection nozzle, and a laser source for supplying a pulsating laser beam, wherein the pulsating timing of the excitation source, the diameter of the orifice and the pulsating timing of the laser source are designed with respect to one another so that the droplets ejecting from the orifice of the injection nozzle have a predetermined speed and an interval between droplets and the target droplets within the droplet stream are ionized by the pulse of the laser beam, and wherein a predetermined number of buffer droplets are supplied between the target droplets so as not to be directly ionized by the pulsing laser beam and the buffer droplets absorb plasma energy radiated from the ionized target droplets so that subsequent target droplets are not affected by the ionization of the precedent target droplets.
Furthermore, Japanese Patent Application Publication JP-P2004-111907A discloses an extreme ultra violet light source including a droplet generator for generating a stream of droplets along an initial path, a steering device for deflecting the droplets from the initial path to a target path, a sensor for detecting the location of the stream of droplets, and an actuator for causing the droplets to be deflected to a target location in the target path by changing the orientation of the steering plate in response to a signal from the sensor.
By the way, in the LPP EUV light source device, it is necessary to form uniform droplets for stable EUV light generation. Here, “uniform” means a state of droplets, after the jet injected from the injection nozzle is divided into droplets, where sizes and shapes of the respective droplets, an interval between adjacent two droplets, etc. are uniform and no satellites are formed near the irradiation position of the pulse laser beam. The satellites refer to minute droplets formed in front and back of the major droplets when the jet injected from the injection nozzle is divided into droplets.
For this purpose, vibration must be provided with appropriate amplitude and frequency to the vibrator for providing vibration to the injection nozzle. However, no mechanism has been disclosed for forming droplets in consideration of amplitude and frequency of the vibrator.
FIG. 9 is a schematic diagram showing a general structure of a droplet generation injection nozzle. The droplet generation injection nozzle includes an injection nozzle 1 for injecting a target material and a vibrator 2 for providing vibration to the injection nozzle 1. A pipe 3 for supplying the target material to the injection nozzle 1 is provided to the injection nozzle 1. Further, a vibrator power supply 4 for generating a voltage applied to the vibrator is connected to two terminals 2a and 2b of the vibrator 2. The vibrator 2 is supported by a supporting part 5 fixed to a vacuum chamber.
The vibrator 2 used for forming droplets itself has a capacitance component (C) and an inductance (L), and operates as one element of an electric circuit as shown in FIG. 9. Such an element is connected to a cable and incorporated within an EUV light source device, and therefore, the vibrator is affected by a wiring capacity, a wiring inductance, etc. On this account, the magnitude of the voltage actually applied to the vibrator 2 changes from the magnitude of the voltage set in the vibrator power supply 4 according to the frequency of the voltage. Thereby, the vibration amplitude of the vibrator 2 dependent on the voltage value also changes.
Thus, when the frequency of the voltage applied to the vibrator is changed, the voltage applied between terminals of the vibrator, i.e., the vibration amplitude of the vibrator varies. Accordingly, there has been a problem that droplets in desired sizes at uniform intervals can not be obtained. Especially, in a piezoelectric element or the like as the vibrator having resonant frequency in high frequency bands, variation is large in the applied voltage around the resonant frequency, which becomes one of main factors inhibiting the generation of uniform droplets. Further, excessive injection nozzle vibration due to resonance is also generated around the resonant frequency band of the entire droplet injection nozzle including the vibrator, and therefore, the generation of uniform droplets is inhibited.